No -- if the passenger holds a return ticket, it would be fair to lift the rebooking restriction for this emergency situation. It's not fair that the airline is able to make money out of this situation (sell a walk up fare, cancel his original ticket and re-sell his original return seat).
HTB.
I guess it also depends if you have status on the airline. I've made same day changes to my ticket and it was cost of fare difference (based on RT) plus change fee. I agree that an airline shouldn't make a profit on a customer's misfortune.